The main point for me is that you don't only need a good definition of left (/right), but you also need to keep your audience in mind.

If you ask me "what type of person doesn't know what 'left' means", my answer would be "either a child or a foreigner who just started learning the language". For that audience, even saying "the side where your heart is" (like some other comment suggests) would require knowing what "heart" means, which might not be a good assumption for this specific audience.

So how would you propose to explain which side is which?

Use an asymmetry of the human body. Left is on the same side as your heart.

Which is again, only most of time (cf: dextrocardia).

Not as common as left-handedness (one in ten), but still in the order of one in ten thousand people.

Your heart is squarely in the middle. You feel the heart beat on the left because the ventricle is larger.

Left: Face the noon sun. Lift one arm to point at where it was first seen this morning.

Edit: works where I live. YMMV. Adapt as needed (which I thought about saying but decided wasn't needed here.)

> Left: Face the noon sun. Lift one arm to point at where it was first seen this morning.

That...fails to be generally accurate pretty badly.

As a quick example: north vs south hemisphere swaps this.

That's approximately correct, though It think it's most actually “north or south of the subsolar point at noon on the day in question”.

North and South of the tropics will give either consistently right or consistently wrong answers, but within the tropics you'll get different answers on different days.